1. Field of the Invention
The present invention relates to a method of suppressing disturbing portions of the Doppler frequency spectrum in a radar receiver with coherent integration.
2. Description of the Prior Art
The Doppler spectrum of interference resulting from reflection from moving reflectors, in particular interference due to weather or sea conditions ("clutter") is not known and so such interference must be adaptively suppressed in radar apparatus. Filters having a fixed transmission function, like the known circuits for permanent echo suppression (so-called MTI filters) fail for this purpose. At present, two methods for adaptive suppression of disturbing echoes are known. Both methods are applied to the echoes directly, i.e. before any coherent integration. With the so-called "clutter-locking" method, the mean Doppler frequency of the disturbance spectrum is estimated and used to shift the blocking band of a conventional MTI filter. This can be achieved by changing the frequency of the coherence oscillator or by approximate phase-shifting of the echo values. Details of this technique is provided in the "EASCON 73 Record" (1973) IEEE, PP. 170-176.
This method has the disadvantage that in order to suppress disturbing echoes, one is dependent upon transmission function of the permanent echo suppression filter (MTI filter). One shifts the blocking band to the mean frequency of the interference, but cannot adapt any further to the pattern of the interference spectrum, in particular the band width. The method fails completely when the interference is made up of a number of components of varying mean frequency overlaid. The aforementioned method can be applied in radars with continuous scanning. When used in radars having progressive scanning, such types of filtering methods have the disadvantage that the number of echoes is reduced by the number of filter coefficients. This leads to a shortening of the coherent integration that normally follows.
In another method, the azimuthal correlation of a disturbance area is estimated, and from this estimation a transverse filter is established for suppressing the interference.
This filter requires a relatively complicated algorithm and fairly precise computation (about 12 bit) to work out the filter coefficients from the estimated interference correlation.